In electrographic printing and copying, toner compositions are applied to an electrostatic latent image formed on a dielectric surface in order to develop the image. The dielectric surface may be a coating on a sheet or a web of paper to which the toner is applied. Alternatively, the dielectric surface may be the charge retentive surface of a drum, belt or the like from which toner applied thereto is transferred to a sheet or web of plain paper. The electrostatic latent image may be established through electrostatic induction by a charged writing head, by ion projection, or through photoconduction, as in electrophotographic copiers. Typically, the toner composition is a liquid toner composed of pigments or dyestuffs combined with a plastic or resinous binder, hereafter called "solid pigment particles" or "colorant", with very small amounts of added charged control agents, and dispersed in a large volume of liquid dispersant, primarily composed of a solvent. One common solvent used in liquid toners is an isoparaffinic hydrocarbon available under the trademark Isopar from the Exxon Corporation.
Multi-color electrostatic printers typically store liquid toner in supply tanks, one for each desired color, and selectively dispense the toner to one or more applicators as it is needed. Usually any excess toner is returned to the appropriate supply tank for reuse. Because pigment particles are deposited in the printing process upon the latent image, the excess toner returned to the supply tanks quickly dilutes the supply of toner until it becomes so dilute that it must be replenished. A concentrated form of colorant is periodically added to the supply tank to restore the colorant removed by the toning process. Liquid toners have a very delicate chemical balance which is easily upset by aging, excess replenishment, contamination, color intermixing, selective constituent removal during electrophoretic toning, or heavy use. If the chemical balance is lost, poor imaging results and the entire content of the tank or tanks must be replaced.
Poor quality, resulting from a chemical imbalance, is manifest on an image by smearing, streaking, background staining, and loss of color concentration or various combinations thereof. Thus, despite many advantages that liquid toning has over other marking methods, it is necessary to periodically dispose of large volumes of combustible liquid. It is difficult, at best, to dispose of such liquid without posing a risk to the environment.
In prior application Ser. No. 077,104, now U.S. Pat. No. 4,799,452, assigned to the assignee of the present application, a liquid toner recycling system was disclosed for removing the solid pigment particles from fluid dispersant after completion of developing a latent image. The liquid toner is introduced at one end of a region defined between an electrode and a particle-accumulating surface, and the liquid toner is carried through that region. The electrode is biased to repel the solid pigment particles so that the particles are deposited on the particle-accumulating surface. Substantially particle-free fluid dispersant remains and is removed at the exit of the region, while the solid pigment particles continue to travel with the particle-accumulating surface to an area at which the particles are scraped from the surface. The substantially particle-free fluid dispersant is then returned to a supply tank. Likewise, the solid pigment particles, now in a state referred to in the art as concentrate, are returned to the appropriate supply tank. Thus, in a multi-color printer a single supply tank of fluid dispersant may be used, with small volumes of color concentrate being added to the fluid dispersant as desired. For example, a two to four ounce supply tank of concentrate may be used for each color in place of the previously used two gallon tank.
While the above-described system meets the object of liquid toner recycling for permitting use of one tank of fluid dispersant with a number of color concentrates, it has been discovered that further improvement in removing solid pigment particles from color dispersant is possible. In addition, it has been discovered that sparking between the electrode and the particle accumulation surface can be substantially eliminated by arranging the system so that toner or carrier fluid always fills the entire space between the electrode and the accumulation surface. Furthermore, it has been discovered that there sometimes are difficulties in redispersing the rather thick agglomerate to make new toner.
It is an object of the present invention to provide a liquid toner recycling system and method which improves upon the elimination of solid pigment particles from fluid dispersant. A further object of the present invention is to provide a liquid toner recycling system with integral redispersion of the separate particles. Yet another object of the present invention is to provide a liquid toner recycling system with an electrode which does not spark relative to nearby structures.